This invention relates to a radiation curable pressure-sensitive adhesive composition having high cohesion at low temperatures and excellent adhesiveness.
Heretofore, in order to exhibit desired adhesive properties such as cohesive force and peel strength in pressure-sensitive adhesive tapes, it has been necessary to use a polymer having a relatively high molecular weight and viscosity (e.g., natural rubber, synthetic rubber, etc.). Therefore, in coating, the polymer should be applied to a substrate in the form of a solution in an organic solvent. Further, since a large amount of the solvent used is allowed to evaporate, a long period of time is required for a drying step after the coating. Moreover, solvents used for the above mentioned purpose are volatile and flammable and many of them have harmful effects on human body, so that their use often causes a fire or various environmental pollution problems. Further, a solvent-recovering apparatus which is regarded as necessary from the viewpoint of economics and preventing the above-mentioned environmental pollution is generally expensive and requires a wide space for its provision. Accordingly, so-called solventless pressure-sensitive adhesives have been noted in recent years from the viewpoints of saving-resources, saving-energy and preventing-environmental pollution. In order to make solventless adhesives, there have been used emulsion type, hot melt type and the like adhesives. Particularly, a radiation curable pressure-sensitive adhesive applying a liquid oligomer having one or more unsaturated double bonds in its molecule is now of interest. This is because the radiation curable pressure-sensitive adhesive does not contain, in principle, an organic solvent which causes defects of the above-mentioned solvent-type pressure-sensitive adhesives, or even if it contains the solvent, the amount of the solvent is very small. Further, there are the following advantages: (1) since radiation which is actinic energy rays is used, the curing (polymerization) reaction is fast, (2) since the curing reaction proceeds only when irradiated with radiation, the pot life can be controlled freely, (3) no large dryer is necessary for the production, etc.
But even the radiation curable pressure-sensitive adhesive has the following disadvantages. The polymerization (curing) reaction generally proceeds by addition of radicals generated by irradiation to unsaturated double bonds, but in this case the reaction rate is so fast that excess crosslinking easily takes place, which results in raising the glass transition point. Therefore, the formed adhesive layer film becomes brittle, and in an extreme case, cracks are formed due to large polymerization shrinkage. When adhesive layer film properties are measured in such a state, the film has large breaking strength but hardly shows elongation and rubber-like properties. On the other hand, when the absolute amount of unsaturated double bonds in the molecule is reduced in order to suppress the excess crosslinking, the reaction proceeds locally, which results in failing to obtain sufficient cohesive force as a whole.
As mentioned above, it as difficult to suppress the curing reaction suitably while maintaining adhesive properties as a pressure-sensitive adhesive.
In order to solve such problems, some of the present inventors have proposed a pressure-sensitive adhesive comprising a radiation curable oligomer, a chain transfer agent and a polar group-containing monomer in order to carry out the curing reaction uniformly and to enhance interfacial force (Japanese Patent Appln No. 128224/82, Japanese Patent Examined Publication No. 25070/85). But even such a pressure-sensitive adhesive has a problem of lowering peel strength in a cold district or in the wintertime.